1. Field of the Invention
The present invention relates to a high tenacity fiber for reinforcing plastic composites and to the composites themselves wherein the reinforcing fiber, selected from the group consisting of polyester, aliphatic polyamide and combinations thereof, has been treated with a composition comprising an aqueous solution of carboxyl-terminated, oil-free alkyd resins. More particularly, the present invention relates to a high tenacity, high elongation, low shrinkage polyester fiber which has been treated with a composition to enhance its stiffness and interfilament coherency to permit high speed cutting of the fiber, especially for use in reinforcing a composite wherein the resin matrix comprises unsaturated polyester resin or other thermosetting or thermoplastic resins. The reinforcements can be used in bulk (BMC) and sheet molding compounds (SMC), filament winding, pultrusion, spray-up and hand-layup, and are especially useful in sheet molding compounds and spray-up applications due to ease of cutting.
2. The Prior Art
A composite consists of basically three major elements: the resin matrix, the reinforcement dispersed in the matrix, and the reinforcement-resin matrix interface. Synthetic fibers in staple or filamentary form, and fabrics produced therefrom, are known for polymer reinforcement. Typical of the fibrous reinforcements are glass, polyester, polyamide (nylon and aramid) and polyolefin fibers. Conventional matrix resins include thermoplastics, such as nylon and polyolefins, and thermosetting materials, such as epoxy and unsaturated polyester resins. Since the primary function of the fiber matrix interface is to transmit stress from the matrix to the reinforcing fibers, the chemical and physical features of the interface are critical to the mechanical properties and end use performance of the composite. The compatibility between the reinforcing fiber and matrix is then a determining factor in the load sharing ability of the composite. Fiber coatings/binders have been used to enhance the compatability of the reinforcing fibers and resins with which they are to be used. See, for example, U.S. Pat. No. 3,637,417 to Green, hereby incorporated by reference. It is known to utilize silane coupling agents to bond dissimilar materials such as organic polymer and fibrous glass in the field of reinforced plastics. See, e.g., Green, supra, U.S. Pat. No. 4,158,714 to Brichta et al., and U.S. Pat. NO. 3,658,748 to Andersen et al., and Marsden and Sterman, HANDBOOK OF ADHESIVES, Second Edition, 40, 640 (1977), all of which are hereby incorporated by reference.
Polyester fiber inherently has a low fiber bundle integrity (fiber bundle integrity is the degree to which the individual filaments adhere to each other). The main benefit of a low integrity fiber is that it enables good dispersion of single filaments throughout a resin matrix. This even distribution results in a homogeneous reinforced composite, a direct result of which is an improved cosmetic appearance. However, in operations where high speed cutting, on the order of 500 feet/minute (about 150 m/min) or higher, of the fiber is required, e.g., SMC'S and spray-up, a low integrity fiber tends to fluff or cottonball and jam up the cutter. A high integrity fiber, readily cut at high speed and which does not ball-up within the reinforced composite, is therefore desirable.
It is also conventional to substitute organic synthetic fibers in whole or in part for glass fibers in reinforced plastics. Some advantages are pointed out in U.S. Pat. No. 3,639,424 to Gray et al., hereby incorporated by reference, wherein heatset polyester staple is used to reinforce thermoplastic/thermosetting polymers for improved impact strength. Heatsetting the fiber allegedly permits uniform fiber dispersion in molded products.
The use of chemically modified polyester fabric as an auxiliary reinforcing agent with glass fibers for thermosets, including polyester, vinyl ester and epoxy, for improved impact resistance and flexural strength over straight glass reinforcement is disclosed in Plastics World Magazine, November, 1980, Volume 38, No. 11, page 102, hereby incorporated by reference.
The present invention is directed towards improving compatibility between a resin matrix and the reinforcing fibers to thereby enhance the reinforced composite properties.